1. Field of the Invention
This invention relates to special cationic layer compounds having a specific BET surface of at least 50 m.sup.2 /g which are modified with polyols, epoxidized esters, full and partial esters of polyols and/or phosphites, to a process for their production and to their use as co-stabilizers for halogen-containing synthetic resins stabilized with calcium and/or zinc salts.
2. Discussion of Related Art
It is known that halogen-containing synthetic resins or molding compounds produced from them tend to degrade or decompose on exposure to heat or ultraviolet light. To counteract this, heavy metal compounds based on lead, tin, barium and/or cadmium are normally used. For reasons of factory hygiene, however, there is a need to replace these thoroughly effective stabilizers by less health-damaging materials. Possible alternative stabilizers to the heavy metal compounds are, for example, calcium and zinc soaps, but unfortunately they do not perform as well as the heavy metal compounds so that co-stabilizers have to be used in order to enhance their stabilizing effect.
German patent DE-C-30 19 632 describes the use of hydrotalcites for inhibiting the thermal or ultraviolet degradation of halogen-containing thermoplastic resins. This patent specification discloses test results which show that, when readily commercially available hydrotalcites are incorporated, for example, in vinyl chloride resins, they accelerate the dechlorination of the resins on heating or even cause decomposition, blackening or foaming of the resins. In addition, it was found that these hydrotalcites show poor dispersibility in the resins and adversely affect the rheological properties of the resins during molding and also the appearance of the molded products obtained. These test results are attributed to the small crystal size of the usual hydrotalcites and to the large specific BET surface of at least about 50 m.sup.2 /g and the coverage of the hydrotalcite particles with water. Accordingly, it is proposed in German patent DE-C-30 19 632 to use hydrotalcites which have a large crystal size and a specific BET surface of no larger than 30 m.sup.2 /g and which may optionally be coated with an anionic surfactant, such as sodium stearate. European patent application EP-A-189 899 also describes resin compositions containing hydrotalcites having specific BET surfaces below 30 m.sup.2 /g. It is known from this European patent application that the hydrotalcites can be modified with higher fatty acid esters, anionic surfactants and coupling agents of the silane or titanium type in order to improve the compatibility of the hydrotalcite with the resins. According to European patent application EP-A-189 899, the hydrotalcites are said to be modified by mechanical mixing of the hydrotalcites with the modifying agents in pure or dissolved form.
However, cationic layer compounds having such small specific surfaces can only be obtained at considerable expense because the crystallization of the solid from the aqueous phase has to take place in a pressure reactor at temperatures well above the normal boiling point of water. This involves considerable outlay on equipment and leads to an unsatisfactory volume/time yield on account of the long crystallization times. In addition, the use of hydrotalcites having small specific surfaces is attended by the disadvantage that hydrogen chloride given off during the thermal decomposition of vinyl chloride resins can only be trapped over a very small surface.
European patent application EP-A-207 811 describes monodisperse cationic layer compounds modified with polyethylene glycol or glycerol which are supposed to be used as thickeners for water-based drilling fluids. It is proposed in this patent application to add the modifying additives to the cationic layer compounds .already formed before the drying step in order to improve their dispersibility in water. However, modified cationic layer compounds produced in this way have been found to be virtually unuseable as co-stabilizers in halogen-containing synthetic resins.